1. Technical Field
The present invention relates to an electrophotographic image recording apparatus and more particularly to such an image recording apparatus which can easily determine whether a drum unit is appropriately mounted at a predetermined position in a main body of the image recording apparatus.
2. Background Art
A conventional electrophotographic image recording apparatus includes a process unit which is comprised of a photosensitive drum, an electric charger , a developing unit and other components. Generally, the process unit is removably mounted in a main body of the image recording apparatus. If the process unit is designed to be able to make 10,000 copies, an associated counter is used to determine whether longevity of the process unit expires: it provides a user with information about the remaining life of the process unit. For example, when the total copy number counted by the counter reaches a certain value, an associated display indicates a message or sign for exchange of the process unit. Therefore, the user is not required to pay attention to a life of each of the components of the process unit such as the photosensitive drum and the developing unit. Rather, the user only needs to replace the existing process unit with a new one when the display unit indicates an "Exchange Process Unit" message or sign.
Although the components of the process unit such as the photosensitive drum and the developing unit of the process unit are expendable components, their longevity are different from each other. Conventionally, when one of the components of the process unit reaches its life limit, the entire components of the process unit should be replaced since the components are integrated to a single process unit. In view of this drawback, there is a proposal which allows independent installation of the photosensitive drum and the developing unit. These two parts are separately mounted in the main body of the image recording apparatus and independently replaced with new ones according to their own life limits.
However, if the photosensitive drum unit and the developing unit are separately installed in the main body of the image recording apparatus, these two parts are respectively lifted up when jamming occurs in the main body of the image recording apparatus. This is troublesome. In addition, if the developing unit is first mounted on a main frame work of the image recording apparatus and then the photosensitive drum unit is mounted to the main frame work, appropriately positioning the photosensitive drum unit relative to the developing unit is difficult since there is no easy way to know if the relative positioning between the photosensitive drum unit, the developing unit and the apparatus main frame is pertinent. To deal with this problem conventionally, a projection is formed on a surface of a lateral wall of the photosensitive drum unit and a photoelectric sensor is provided to detect whether the projection is in place or not. If the photoelectric sensor determines that the projection is in a predetermined position, then the photosensitive drum unit is assumed to be appropriately mounted on the main frame of the image recording apparatus. However, this arrangement increases the number of the parts and raises a manufacturing cost. The photoelectric sensor is an expensive part.
In general, the photosensitive drum of the image recording apparatus has a surface formed of two layers: an outer photoconductive layer and an inner conductive layer. When the photosensitive drum is installed in the main body of the image recording apparatus, the inner conductive layer should always be grounded via a metallic portion (e.g., metallic frame) of the main frame of the image recording apparatus. Grounding is, for instance, established by contact between a shaft extending from the photosensitive drum and a hub-like portion of the metallic frame for receiving the drum shaft. A user removes the drum unit from the apparatus frame when paper jamming occurs or a life of the drum unit is assumed to have expired. However, if the drum shaft is not perfectly received in the hub-like portion of the apparatus frame when the drum unit is placed in an original position again after eliminating jamming or a new drum unit is installed in place of the old drum unit, the grounding becomes insufficient so that a desired electrostatic latent image is not created.
One example of conventional arrangements is disclosed in Japanese Patent Application, Laid-Open Publication No. 6-83251.